Keyed automatic gain control with keying pulse limiter



K. R. WENDT Oct. 6, 1953 KEYED AUTOMATIC GAIN CONTROL WITH KEYING. PULSELIMITER zsheets-sheer 1 Filed Aug. 26, 1948 :inventor Gitan-neg K. R.WENDT Oct. 6, 1953 KEYEO AUTOMATIC GAIN CONTROL WITH KEYING PULSELIMITER Filed Aug. 26, 1948 l.2 SheebS-Sheet 2 SSG,

Gttomeg Patented Oct. 6, 1953 KEYED AUTOMATIC GAIN CONTROL WITH KEYINGPULSE LIMITEE` Karl Binner Wendt, Hightstown, N. J., assignor to Radio`Corporation of America, a corporation of Delaware Application August26, 1948, Serial No. 46,254

1 Claim. l

This invention relates to automatic gain `control systems for use inradio receivers adapted to receive image signals having synchronizingand black level (blanking) information included therein and moreparticularly for use in present `day television receiver systems.

The present invention deals more particularly with keyed automatic gaincontrol systems of 4the type requiring the application of constantamplitude keying pulses .and is particularly use- 4f-ul in connectionwith `a-n inverted form .of keyed AGC system .of the type, for exampledescribed by Karl R. Wendt in U. S. Patents 2,637,772, issued May 5,1953 and 2,586,193, issued February 19, 1952, which 4operate to controlthe degree of amplication .of the video .signals aiorded by thetelevision receiver in accordance with the intensity of the radiocarrier received during the synchronizing or blank'ing intervals. ln.order to achieve the action described by the aforementioned referencesuse is made of a gated amplier or .conduction y,device which responds toa series of applied keying pulses. More particue larly in the aboveidentined S. Patents, ,a 'source of such keying pulses is indicated asbeing conveniently available `from the deilection circuit of thetelevision receiver in which the automatic gain control is incorporated.As will be seen hereinafter, subsequent to a brief .consideration ofautomatic gain control .circuit character- 5 istics, such a source .ofkeying pulses although eminently suitable from a timing standpoint, doesin some instances admit to certain inaccuracies in circuit ,operation.due to variations in the amplitude of the keying pulses inherentlyVproduced thereby. More particularly is this true in connection with atelevision receiver incorporating combination 4deiection circuits whichnot only provide deflection energy for the kinescope electron beam butalso supplies energy for the actual acceleration of .the beam within thetube.

It is 4commonly known that the automatic gain control circuits for usein television receiving equipment differ greatly from the morefrequently encountered automatic gain control circuit embodied inreceivers for .sound broadcast signals. In the instance of the usualbroadcast receiver designed for the reception of amplitude modulatedcarriers, it is deemed adequate that the automatic gain controlpotential be produced by electrical information gleaned from the averagecarrier intensity of the received radio signals. Clearly such anautomatic gain control circuit would not be satisfactory for controllingthe gain of television receiver video channels as the average signalstrength of the radio frequency Y,carrier is a function of the averageimage or picture brilliance sometimes referred to as background level.As pointed out in the aforementioned vapplications rby Karl R. Wendt,`development of an automatic gain control voltage in accordance with theaverage signal ,strength of the received radio carrier would. cause .thegain to be changed not only in accordance with the signal intensityvaria-tions .of the received radio carrier due to `undesirable fading orother atmospherical phenomena, but also in accordance Ywith averagepicture brilliance of the image being transmitted.

Rad-io transmitted negative modulated television signals normallyinclude blanking pulses or black level information Which data istransmitted between each image line in combination with the linesynchronizing pulse. This line sync pulse is most commonly superimposedupon the lblack level signal and these data are transmitted at maximum,carrier intensity. or 10.0 percent `carrier amplitude, While the 'blacklevel .or blanking pulse is transmitted .at approximately percent of thefull carrier amplitude. The blanking impulse or black level inaccordance with present R. M. A. television 4synchronizing wave formstandards is of the order lof i6 to 18 percent of a line interval withthe sync i-mpulse having a period of approximately 8 percent of a lineinterval. Sync impulses when superimposed upon the blanking or blacklevel signals are stationed between the extremities of the blankinginterval so as to form what is commonly termed a front porch and a backporch on the pedestal-like waveform structure formed by the combinedsync signal and blanking impulse. The front porch interval isapproximately 21A; percent of the line interval and` represents the timebetween the leading edge of each black level signal and the leading edgeof the line sync pulse, Whereas the back porch interval of approximately6 percent of a line interval, is equi-valent to the time intervalbetween the end of the line sync pulse and the termination of the blacklevel or blank .out signal.

The amplitude of the radio frequency carrier as previously brought outis held constant during the transmission of all blanking and syncimpulse information. During the transmission of image intelligence ofthe television signal, that is, during each line interval betweensuccessive blank signals, the average amplitude of the transmitted radiofrequency `carrier isa function of the average light contained in thetelevision image. Accordingly, if the reproduced irnage is to bepredominantly dark, the average carrier amplitude will necessarily begreater would be the case if the background level of the image wereconsiderably lighter, such action of course is true only in the negativesystem of transmission wherein white picture information is transmittedat a lower carrier level than black level information. it is expedientin order that the control of the gain of the receiver be in accordancewith the proper aspects of the carrier, that the automatic gain controlpotential be developed such that its magnitude is a function of theintensity of the received carrier of the television signal during theblanking cr sync intervals only and, as hereinbefcre brought out, notduring the transmission of the picture or line information.

In order to extract carrier strength information from the televisionsignal at periods corresponding to blanking or sync intervals use may bemade of keyed circuits which translate signal information only duringthe keyed state of the circuit. 1n this manner, and as more fullydescribed in the aforementioned applications of Karl R. Wendt, supra, anautomatic gain control potential may be developed which is substantiallyfree of noise effects and is to be much preferred over the automaticgain control action derived from simple peak rectifier systems which arequite vulnerable to spurious energy bursts at any time during thereception of the Also with the keyed type of AGC a properly arrangedcircuit will ideally display 20 Vtimes the noise immunity possible inunkeyed automatic gain control circuits. However, in some forms of AGCsystems heretofore proposed, the AGC potential as developed by thesystem is rendered a `function of the amplitude of the applied keyingpulses. Therefore, should the amplitude of the keying pulses vary duringre- 'ception of a signal, misinformation would be provided to the signalamplifying circuits to produce an undesired change in signal intensityat the image viewing device. In the instances where the keying pulsesare derived from the deflection circuit and said deflection circuit alsoprovides energy for acceleration of the electron beam within thekinescope, a change in received picture background level producescorresponding changes in loading upon the denection circuit and causesthe derived keying pulses to change in amplitude in accordance withpicture content. It is evident that such conditions are extremelyunfavorable and substantially reduce the overall picture qualityproduced by the television receiver.

According to the present invention the deleterious effects of keyingpulse amplitude variation in keyed AGC systems are substantiallyeliminated by means of providing limiting means to restrict theamplitude variations of the keying pulse as it is applied to the keyingcircuit. The present invention provides such limiting action inconnection with receivers using keying pulses derived from the kinescopedeflection circuit, through the employment of very little additionalequipment and consequent cost of supplemental circuit structure.

Therefore, it .is the purpose of the present invention to provide animproved keyed automatic gain control circuit, which is simple inoperation and suitable for embodiment in standard television receivers.

fi It is another purpose of the present invention to provide keyingpulse energy for keyed automatic gain control circuits finding use intelevision receivers such that the action of the automatic gain controlcircuit is substantially independent of variations in the operation ofthe source of keying pulse energy.

It is another object of the present invention to extract andproperlycontrol automatic gain control keying pulse signals fromtelevision receiver deflection circuits of the type providing bothkinescope electron beam accelerating energy and electron beam deflectionenergy, whereby the operation of the automatic gain control circuit isrendered substantially independent of picture background information.

The novel features which are believed to be characteristic of thepresent invention and set forth in the appended claim, the inventionitself however as to both its organization and method of operation willbe best understood from Vthe teaching of the following descriptionespecially when considered in connection with the accompanying drawingswherein:

Figure l is one embodiment of the present invention as applied to aconventional television receiving circuit.

Figure 2 shows another form of practicing the present invention inconnection with the receiving circuit shown in Figure 1.

Figure 3 shows another embodiment of the present invention as practicedin connection with a conventional television receiver of a type depictedin Figure l.

Figure 4 illustrates still another embodimen of the present invention asadopted to the receiving arrangement shown in Figure l.

in the drawings, like elements are assigned like reference characters.

Referring now to Figure l, there is indicated in block I0 certainwell-known components of a conventional television receiver includingthe R. F. amplifier, the oscillator, the rst detector or converter, andthe intermediate frequency amplier. The signals are picked up by antennal2 and through the medium of transmission line I4 are applied to theinput of the receiver as shown.

Typical and suitable components of the receiver as well as othercomponents employed in the practice of this invention are shown anddescribed in the book entitled Principles of Television Engineering byDonald G. Fink. The output of the intermediate frequency amplifierincluded in block Iii is suitably connected through path I6 through thevideo demodulator I8, which is also shown in block form. The demodulatorI8 demodulates the video intermediate frequency carrier to produce theimage signals including blanking and sync information and subsequentlyapplies the same via a direct current coupling path to grid 22 of vacuumtube 24. The vacuum tube 24 is connected for operation as a videofrequency amplifier being typically provided with frequency compensationby means of elements 26, 28 and 30 connected in series with thepolarizing potentials for the anode 32. Also connected in series withthe polarizing potentials path for the anode, is decouplingresistor-,Sll which acts in conjunction with by-pass 35 to isolate videoamplifier plate current changes from voltage polarizing potentialterminal 38.

Establishment of the plate current -characteristic reg-ion over whichthe video amplifier tube 24 .nected with ground potential. i 16 isconnected with a positive source of potential indicated by +s. g., whilepolarizing potential for is to operate is accomplished throughadjustment of Vthe tap 40 on potentiometer 42, the potentiometer beingconnected to form a bleeder to ground across a bias supply potentialmade available at terminal 50. The video signal including synchronizinginformation applied to the grid 52 of the kinescope I54 is also appliedto the input of the sync separator and amplifier 56 via circuit path 58.In accordance with conventional television receiver design the output ofthe sync signal amplifier 56 is connected with the vertical andhorizontal deflection driving generators 60 `and 62 respectively. Thefunctions of these de- `ilections drive generators is to supply asuitable form of sawtooth voltage for conventional deflection of theelectron beam in the kinescope 54. The output of the vertical deflectiondrive generator 60 is ythen applied to the vertical `deflection outputstage 64 which is indicated as being connected to the vertical deiectionwinding yoke 60 by terminal designations Y-Y, common to both elements.Whereas, the compounds of the deflection output stage 64 areconventional in nature and hence clearly representable in block diagram,the horizontal deection output stage connected for excitation of thehorizontal deflection yoke 69 is shown in more detail as it is to alsoserve, as hereinafter more fully described, in the capacity of a controlfor the subject AGC system as well as a pulse step-up power supply foracceleration of the electron beam kinescope 54.

Accordingly, the output of the horizontal deflection drive generator 62is applied to the grid `'I0 of the vacuum tube 12, suitable grid returnimpedance being illustrated by resistor 14 con- The screen grid theanode 18 is supplied through deflection output 4transformer primarywinding 80, from a source of positive potential having a terminalindicated at 82. In a lconventional manner the deflection signal for thedeilection yoke winding B9 is supplied from the secondary winding 84 ofthe output transformer 86 by means of a suitable connection between theterminals of the elements 69 and 8=4 indicated at X-X. Across thesecondary winding 84 there is provided a damping circuit 90 which isthere placed to improve the eiiiciency and waveform produced by thedeflection circuit.

In accordance `with present day practice an autotransformer type winding92 is connected with the primary Winding 80 and also to the anode `94 ofhigh voltage rectifier 9B. The cathode 98 of the rectifier -96 isproperly heated by f energy derived from the deflection signal throughthe medium of secondary winding on the output transformer 86. With thehigh voltage rectifier 96 so connected, the yback impulse produced inthe primary winding 80 of the horizontal output transformer 86 ismagnified in amplitude through the action of the autotrans- `formerwinding 92 and rectified to p-rovide a high unidirectional acceleratingpotential for application to the accelerating anode |02 of the kinescope54 via circuit path |04. A capacitor is shown connected from theaccelerating anode |02 to ground to effect a storage and iilteringaction in cooperation with resistor |06.

It may be Well noted at this time that with such an arrangementvariations in beam current within the kinvescope 54 will reflect throughthe rectier 96 and subsequently through the autotransformer winding 92,connected with the primary winding 80, to reduce the amplitude of de-`flection voltage availableat the output of secondary winding 84. Withthis in mind it is noted that also assocaited with the horizontal outputdeiiection circuit transformer is an auxiilary secondary winding |08which operates to supply a series of pulses ||0 derived from thekickback phase of horizontalldeflection circuit operation. Consequently,these pulses Will also undergo amplitude changes with varying loadconditions imposed upon the deflection circuit as a result of changes inbeam current in the kinescope 54. As will be made clearer hereinafter,the pulses so derived from Winding |08 are applied as keying pulses toan automatic gain control circuit.

According to the present invention, the keying pulses ||0 derived fromthe auxiliary secondary winding |00 are `applied through coupling-capacitor and circuit path H2 to the anode ||4 of the diode H6.Theanode ||4 is also placed at some positive potential by its connectionto volts at power supply terminal ||8 through load resistor |20. Thediode cathode |22 is then appropriately placed at some negativepotential relative to the anode ||4 by its connection to cathoderesistor |24 connected to tap |26 of the potentiometer |28. Adjustmentof the potential of the cathode |22 may be made by positioning the tap|26 on the potentiometer |28 since the potentiometer in combination withthe resistor |30 forms a bleeder to a ground across the +150 volt powersupply terminal |8. Inasmuch as the diode is then placed underconditions for conduction, the pulses ||0 will be conducted therethroughand developed across the cathode load resistor |24 to produce a seriesof negatively extending amplitude limited ypulses |0A on the cathode |32of AGC triode |34. The Value of the limiting action so obtained will bemade manifest upon consideration ofthe operation of the AGC' as a whole.However, it :can be clearly seen that as the negatively extending keyingpulses ||0 become sufliciently great in amplitude to swing or to drivethe anode ||4 to a` potential equal to, or more negative than thecathode |221, the diode ||6 will then establish a limiting action due tonon-conduction. i

The operation of the automatic gain control circuit Will be shown forthe purpose of explanation as being of the type disclosed in theaforementioned U. S. patents supra. It is to be noticed that the videosignal at the output of the Video amplifier 24 applied to thesynchronizing separator amplifier 56, is obtained through the samecircuit path 58 through which energy is applied to` thegrid |38 of theAGC triode |34. The form of the video signal voltage here applied byamplifier 24 is illustrated by the curve |40 shown in time relationshipwith the application of the amplitude limited keying pulses to thecathode |32 as derved from the horizontal output transformer auxiliarywinding |08. Accordingly, it is seen that the keying pulses |0A so`applied to the cathode |32 of the vacuum tube |34 are not only in adirection to drive the cathode negatively during their generation butare timed relative to the received video signal |40 to occur during thefback porch interval |4| of the blanking and synchronizing signalpedestal combination. Since the D. C. potential applied to the grid 52of the kinescope 54 is positive with respect to ground by an amountequal to the difference between +150 volts and the voltage drop `acrossthe elements 28, 30, and 34, the cathode 53 of the kinescope 54 and thecathode |32 of tube |34 must be placed at'a'positive potential in excessof their associated grid potentials. In the case of the kinescope 54, yacon- Ventional form of brightness control is shown comprisingpotentiometer |45 connecting in series with the bleeder resistor |46 anda positive potential source |48 of +150 volts. Therefore, by adjustmentof the tap |50 of the potentiometer |45, the negative bias on the grid52 of the kinescope 54 may be varied to control the brightness orintensity of the beam within.

In the case of the AGC vacuum tube |34, the cathode |32 has been shownas being connected to a positive potential which is considerably inexcess of that existing on the grid |33 by the connection of the cathode|32 with the upper end of the load resistor or cathode resistor |24,which is in turn connected with the potentiometer tap |25. Consequently,positioning of the tap |26 on the potentiometer Will not only permitadjustment of operating potential of the diode |||i but also allow avariable net negative bias to be applied to the grid |38 of the AGCtriode. In normal operation of the present exemplary form of automaticgain control this `level is established so as to produce plate currentcut-off in the vacuum tube |34 during the absence of the amplitudelimited negatively extended keying pulses ||A.

Under such bias cut-off conditions if the keying pulses HOA (poled todrive the cathode |32 negatively) are of suncient amplitude they willovercome the negati-ve cut-off bias on the tube |34 and establish platecurrent conduction during intervals of their production. Correspondiingly, then With the arrangement shown herein, the keying pulses Illaoccur once during each blanking interval and are phased with the backporch portion thereof so that the tube |34 will be rendered conductiveduring this back porch period. Inasmuch as the blank out signal |4|extended in a negative direction on the grid |38, the net plate currentpulses in vacuum tube |34 due to the combined effect of signals |40 andIDA Will be of an amplitude inversely proportional to the blanking levelduring keyed operation of the tube |34. Since the `current pulses in theplate load resistor |52 are inversely proportional to the received blacklevel signal, the negatively extending plate voltage pulses |54 Willthen also be inversely proportional in amplitude to the received videosignal black level (corresponding to the back porch or pedestal portionof the signal). As the pulses ||0A are of constant amplitude, areduction in received signal strength attended by a decrease in theamplitude of the video signal |40 will permit the pulses ||0A to drivethe tube |34 into `a heavier state of conduction and hence producegreater amplitude plate voltage pulses |54. Conversely, an increase insignal strength during the keyed conduction-of the vacuum tube |34 willoperate to reduce the amplitude of the plate voltage pulses Thenegatively extending pulses |54 are then 'applied through couplingcondenser |56 to the anode |58 of a diode |60. The anode |62 of thisdiode and the cathode thereof are connected through suitable loadresistors |64 and |65. The pulses |54 extending in a negative directionthereby cause conduction of the diode |60 with a consequent developmentof a unidirectional voltage across load resistor |64 across which isplaced storage condenser |66. Therefore, the voltage developed acrossthe resistor |64 is proportional to the amplitude of the applied pulses|54 and consequently this potential must vary inversely with receivedvideo signal. Ac-cordingly, the potential so developed across resistor|64 is connected to the grid |61 of the D. C. amplifier |68 having itsanode |10 connected through load resistor |12 to ground potential. TheD. C. amplifier |68 is in turn rendered operative by connection of itscathode |14 through load resistor |16 to a 100 volts made available atterminal |18. This permits development of a suitable AGC voltage acrossresistor |12 which may then be applied to the R. F. and I. F. sectionsof the television receiver through AGC bus |15. It may be noted that theseries condenser resistor combination .|18 is placed across the storagecondenser |66 to provide a conventional damping action and therebydiscourage oscillating or motorboating of the automatic gain controlcircuit.

The AGC `action thus obtained is readily discernible throughconsideration of the various circuit responses under the conditionsattending a reduction in signal strength at the grid 52 of the kinescope54. In suchV case, the video signal as applied to the AGC triode grid|38 would be reduced in amplitude and consequently elfect an increase inthe amplitude of the voltage pulses |54 appearing at the anode of thetriode |34. As a result, the voltage across the storage capacitor |65will increase and thereby cause the negative voltage With respect toground already existent on the grid |61 to become more negative. Thisincrease in the negative bias on the D. C. ampiier |68 Will then causeits output voltage appearing across resistor |12 to become more positiveand thereby apply less negative bias through the AGC bus |15 to the R.F. and I. F. amplifiers included in the block |0. This decrease innegative bias causing an increase in amplier gain consequentlycorresponds to the decrease in signal strength.

Under the conditions cited above if the signal strength should remainconstant and the picture background level become brighter, more energywould be required of the horizontal deilection circuit by the electronbeam with the kinescope 54. Therefore, were it not for the limitingaction provided by the novel application of the diode ||6 to the AGCsystem in accordance with the present invention, the applied pulses ||0Awould necessarily decrease in amplitude as a result of the reduction inamplitude of the pulses ||0 developed across the winding |08. This wouldresult in a decrease in amplitude of the voltage pulses |54 appearing atthe anode of the AGC triode |34 and would in eiTect be the equivalent ofan increase in signal strength. Therefore, the voltage across storagecapacitor |65 would be reduced in magnitude and provide less negativebias to the D. C. amplifier grid |61 which in turn will cause thevoltage at the anode |10 to become more negative. This action reducesthe gain of the receiver, thereby tending to maintain the averageillumination of the kinescope screen at a constant level, which is ofcourse highly undersirable. With the present invention, however, asshown in Figure l if the amplitude of the pulses ||0 is initiallyestablished suliciently in excess of the plate cathode potentialdiierence in the diode H6, such improper action will be obviated throughthe application of a constant amplitude keying pulses |||1A to the AGCtriode |34.

Another method of obtaining a suitable limiting action in connectionwith the specific form `ofautomatic gain control shown in Figure l isillustrated in Figure 2. Here the variable amplitude pulses derived fromthe auxiliary winding |08 on the horizontal output transformer 89 arepoled in a positively extending direction and applied through couplingcapacitor |99 to the grid |92 of limiter triode |94. A suitableimpedance |99 is connected between the grid |92 and the cathode |91 ofthe limiter triode to permit grid current limiting of the applied pulsesH9. The AGC triode |34 is then supplied with a video signal in exactlythe same manner as shown in Figure 1, but the cathode |32 thereof is nowconnected with the anode |98 of the limiter triode |94. Since thecathode |91 of the limiter triode is supplied with a suitable positivepotential by means of tap |26 on potentiometer |28, the limiter triode|94 acts as a variable cathode resistance connection in lieu ofresistance |24 in Figure l. Consequently, if the pulses |0 applied tothe grid |92 of the limiter triode are adjusted to be always in excessof that necessary to drive the grid |92 positive with respect to thecathode |91, grid current through the resistor |96 will limit themaximum degree of conductance exhibited by the triode |94 and henceapply to the cathode |32 constant amplitude keying pulses.

Study of embodiments of Figures l and 2 will reveal that the horizontaldeflection system must supply a certain amount of energy to the AGCsystem in its provision of keying pulses therefor. For example, inFigure 1 keying pulse energy will be dissipated across the resistor |24located in the cathode circle of the AGC triode |34. Again in Figure 2,although the amount of keying pulses of energy required here issubstantially less than in Figure l, it is apparent that grid currentthrough resistor |96 will represent a certain amount of power loss.

The embodiment of Figure 3, however, demands even less keying pulseenergy since no grid current in the limiting triode is caused to flow.Here it can be seen that the keying pulse |0 is applied through couplingcapacitor 220 to the limiting triode grid |92 in a negatively extendingdirection. The pulses l I9 are of substantially greater amplitude thanthat required to drive the triode |94 to plate current cut-ofi.Adjustment of the required pulse amplitude for achievement of cutoii maybe made by varying the value of dropping resistor 222 which appliesoperating potential to the plate |98 of the limiter triode power supplyterminal 224. The bypass condenser 226 places the plate |98 at asubstantially A. C. ground potential. It can be seen then that since thecathode |91 is connected with the cathode resistor 226 which is includedin the cathode circuit of the AGC triode |34, plate current cut-ofi ofthe triode |94 will define the most negative extent of the limitedpulses IIJA appearing on the cathode |32 of the AGC triode. Thus, withproper adjustment of circuit parameters variation in amplitude of thepulses Il] will not be evidenced in the amplitude of the -keying pulses||0A actually keying the AGC' triode |34 and hence will not affectoperation of the AGC system.

The arrangement of Figure 4 utilizing the keying pulses as derived fromthe horizontal dedeation in the negatively extending direction as wasthe case in Figure 3, but however considerably more power is requiredfor the operation of the limiting circuit and the keying of the AGCtriode inasmuch as a diode 230 is employed as a peak saturation type oflimiter. Here the cathode 232 of the diode is connected with the cathode|91 of the AGC triode and the cathode resistor 234 is common to both thecathode circuit of the AGC triode |34 and the load circuit for the diode2.39. The amplitude of the pulses it coupled to the diode cathode 232`through coupling capacitor 240 and resistor 242, are or sufficientamplitude to drive the cathode 232 negatively with respect to thediodeanode 236. Therefore adjustment of the tap |26 on the potentiometer|26, which adjusts the degree to which the anode 239 is placed initiallynegative with respect to the cathode` 232, establishes control over theamplitude of the limited version of the keying pulses applied to thecathode |99 of the AGC triode. If the amplitude of the pulses on anode232 tends to increase the diode 23,0 will be forced into greaterconduction and therefore limit the pulse amplitude appearing at thecathode |99. by an increase in pulse voltage drop across resistor 242.

Although in the description and illustration of the above embodiments ofthe present invention, reference has been made to discrete operatingpotentials for various portions of the circuit arrangements shown, it isto be understood that these potentials are merely exemplary in magnitudeand polarity and that suitable changes therein with proper regard to theteachings of the present invention will provide equally satisfactoryresults. Furthermore, the source of varying amplitude keying pulses hasbeen shown as being provided by an auxiliary winding |08 on thehorizontal deection circuit of the transformer in all or^ the aboveembodiments. It is to be noted that in deflection circuits of the typewell-known to the art, there are numerous terminals rom which suitablekeying pulses may be derived and therefore such an auxiliary windingalthough being conveniently free of any D. C. potentials is merely shownby way of example and its use is in no way intended to limit thepractice of the present invention. Attention is also directed to thefact that although the keyed AGC system herein employed and described inconnection with the use of the present invention is of the inverted typeas disclosed by me in the above cited U. S. patent applications, theutility of the present invention is not coniined solely thereto. Thereare other keyed forms of AGC circuits presently known which willsubstantially benet through the application of this system.

From the foregoing description, it can be seen that the applicant hasprovided a simple, novel, economical and useful improvement in keyedforms of AGC systems and more particularly to those systems in whichkeying pulse information is to be derived from an associated kinescopedeection circuit operating to supply both beam deflection energy as wellas beam accelerating energy.

What is claimed is:

In a television receiver adapted to receive a series of image signalsinterspersed with regularly recurrent black level signals, said receiverincorporating a combination kinescope power supply circuit forsimultaneously supplying kinescope beam deection signal energy as wellas kinescope beam deflection accelerating potential, an automatic gaincontrol circuit comprising in combination a variable conductance device,means maintaining said variable conductance device in a state of minimumconduction, circuit connections to the combination kinescope powersupply for deriving therefrom a series of keying pulses recurring insynchronism with the received black level signals, said pulses havingamplitude charl l acteristics which are a function of the loadingimposed upon the combination power supply, a rst and second pulseamplitude responsive means for controlling the conductance of thevariable conductance device, connections applying the received blacklevel signals to said rst pulse amplitude responsive means, a pulseamplitude limiting circuit to the input of which are applied said keyingpulses, circuit connections applying the output of said amplitudelimiting circuit to said second pulse amplitude responsive means forkeying said variable conductance device into exhibiting a greater degreeof conductance during the active periods of said pulses, and means fordeveloping an automatic gain control potential in accordance with theconductance displayed by the variable conductance device during theintervals in which said keying impulses are active to increase theconductance of said conductance device. Y i

KARL RINNE'R WENDT.

Number Name Date 2,259,538 Wheeler l Oct. 2.1, 1941 2,303,909 BlumleinDec. 1,1942 2,307,375 Blumlein et al Jan. 5, 1,943 2,307,387 Blumlein 11 Jan. 5, 1943 FOREIGN PATENTS Y Number Country Y Date 520,584 GreatBritain Apr. 29, 1940 873,623 France July 15,1942 845,897 France Sept.4, 1939

